Bonded optical element and manufacturing method thereof

ABSTRACT

A second optical element is bonded to a first optical element by forming the second optical element by pressing a second optical element material against the first optical element under a condition free from restriction to the second optical element material in the direction orthogonal to a direction in which the second optical element material is pressed.

FIELD OF THE INVENTION

The present invention relates to optical elements used for opticalsystems and the like, such as picture-taking lenses, optical pickups,and the like and especially relates to a bonded optical element in whichoptical elements different from each other are bonded to each other anda method for manufacturing it.

DESCRIPTION OF RELATED ART

Conventionally, bonded optical elements in which two or more kinds oflenses or prisms are bonded to each other are manufactured in such amanner that optical elements finished in advance by polishing(grinding), pressing, or the like are bonded to each other by means ofan adhesive typified by a UV curing agent. This method, however,involves a step of manufacturing each optical element, a positioningstep of arranging and positioning the two or more optical elementshighly accurately, and a bonding step of applying and curing theadhesive uniformly with air void eliminated, which serve as a bar toincreases in accuracy and productivity of the bonded optical elements.

For tackling this problem, there has been proposed in order to eliminatethe positioning step and the bonding step methods for forming a bondedoptical element by bonding by fusing glass materials.

For example, Patent Document 1 discloses a method for forming acomposite optical element in which a first optical element and a secondoptical element material are disposed between an upper die and a lowerdie, and the second optical element material is pressed by the dies andthe first optical element while being heated at a temperature capable ofdeforming the second optical element material and incapable of deformingthe first element to form a second optical element, thereby integratingthe first and second optical elements with each other.

Patent Document 2 proposes a method including the steps of: forming afirst optical element by heating and pressing a first optical elementmaterial with the use of a pair of first and second dies and a sleevedie; taking out the second die and placing a second optical elementmaterial and a third die; and forming a second optical element byheating and pressing the second optical element material with the use ofthe first optical element and the third die, whereby the first andsecond optical elements are positioned accurately.

In addition, in order to prevent a crack caused due to a shortenedcooling period from being formed, Patent Document 3 proposes a method offorming a glass lens by directly bonding glasses to each other betweenwhich the difference in linear expansion coefficient is in a rangebetween 3×10⁻⁷ and 8×10⁻⁷.

Patent Document 1: Japanese Unexamined Patent Application Publication60-67118

Patent Document 2: Japanese Unexamined Patent Application Publication11-130448

Patent Document 3: Japanese Patent No. 3763552

SUMMARY OF THE INVENTION Problems that the Invention is to Solve

The center thickness of an optical element rather influences the opticalperformance thereof and must therefore be managed highly accurately. Thethickness accuracy of a bonded optical element formed by bonding two ormore optical elements to each other is a sum of the thickness accuraciesof the respective optical elements.

However, Patent Documents 1 to 3 are silent about the accuracy inthickness of the bonded optical element.

The thickness and the weight of each optical element of the bondedoptical element may be managed more accurately than ever for highlyaccurately managing the thickness of the bonded optical element, whichleads to an increase in cost and is therefore not preferable in massproduction.

The present invention has been made in view of the foregoing and has itsobject of managing the center thickness of a bonded optical elementhighly accurately.

Means for Solving the Problems

A bonded optical element in accordance with the present inventionincludes a first optical element and a second optical element bonded toeach other by forming the second optical element by heating and pressingagainst the first optical element a second optical element material,wherein the second optical element is formed by pressing the secondoptical element material against the first optical element under acondition free from restriction to the second optical element materialin a direction orthogonal to a direction in which the second opticalelement material is pressed.

A bonded optical element manufacturing method in accordance with thepresent invention is a method for manufacturing a bonded optical elementin which a first optical element and a second optical element are bondedto each other by forming the second optical element by heating andpressing against the first optical element a second optical elementmaterial, which includes the step of: forming the second optical elementby pressing the second optical element material against the firstoptical element under a condition free from restriction to the secondoptical element material in a direction orthogonal to a direction inwhich the second optical element material is pressed.

Effects of the Invention

According to the present invention, the center thickness of the bondedoptical element can be managed highly accurately with highly accuratemanagement of the thickness and the weight of each optical element ofthe bonded optical element necessitated less than ever.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a bonded optical element in accordancewith an embodiment of the present invention.

FIG. 2 presents schematic sectional views showing a bonded opticalelement manufacturing method, wherein FIG. 2( a) shows a state in whichformation of a second optical element is prepared and FIG. 2( b) shows astate in which formation of the second optical element is completed.

FIG. 3 is a sectional view of a bonded optical element in which a firstoptical element and a second optical element are a spherical biconvexlens and a spherical biconcave lens, respectively.

FIG. 4 is a sectional view of a bonded optical element including a firstoptical element formed by press forming.

FIG. 5 is a sectional view of a bonded optical element in which a secondoptical element and a third optical element are bonded to and integratedwith the upper face and the lower face of a first optical element,respectively.

FIG. 6 is a sectional view of a bonded optical element in which only asecond optical element is subjected to centering.

FIG. 7 is a sectional view of a bonded optical element of which lensface parts of the bonded faces are aspheric.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will be described below withreference to the accompanying drawings.

FIG. 1 is a sectional view of a bonded optical element 7 in accordancewith the present embodiment. As shown in FIG. 1, the bonded opticalelement 7 includes first and second optical elements 4, 6. The firstoptical element 4 is a spherical biconcave lens having an outer diameterof 15 mm, a center thickness of 2 mm, and an edge thickness of 4 mm. Thefirst optical element 4 is made of a material, PBK40 (a product bySumida Optical Glass Inc.) having a refractive index nd of 1.51760, anAbbe number vd of 63.5, a glass transition temperature (transitiontemperature) Tg of 501° C., and a glass softening temperature (a flexuretemperature) At of 549° C. The first optical element 4 is formed bypolishing.

The second optical element 6 is bonded directly to the upper face of thefirst optical element 4 to be integrated therewith. The second opticalelement 6 is a spherical biconvex lens having an outer diameter of 12mm, a center thickness of 4 mm, and an outer peripheral thickness of 1.5mm. Namely, the outer diameter of the second optical element 6 issmaller than that of the first optical element 4. The second opticalelement 6 has a curved outer peripheral face 8 in a substantially curvedform in section protruding outward in the radial direction. The secondoptical element 6 is made of a material, SF8 (a product by SumidaOptical Glass Inc.) having a refractive index nd of 1.68893, an Abbenumber vd of 31.2, a glass transition temperature Tg of 430° C., and aglass softening temperature At of 459° C. Namely, the glass softeningtemperature At of the second optical element 6 is lower than the glasstransition temperature Tg and the glass softening temperature At of thefirst optical element 4.

A method for manufacturing the bonded optical element 7 will bedescribed below with reference to FIG. 2. FIG. 2 presents schematicsectional views showing the method for manufacturing the bonded opticalelement 7, wherein FIG. 2( a) shows a state in which formation of thesecond optical element is prepared and FIG. 2( b) shows a state in whichformation of the second optical element is completed.

First, the first optical element 4 and a second optical element material5 are prepared. The second optical element material 5 has, as shown inFIG. 2, a curved outer peripheral face (a side face) 5 c in asubstantially curved form in section protruding outward in the radialdirection and is smaller in outer diameter than the first opticalelement 4. Next, a lower die 2 is inserted into a sleeve die 3, and thefirst optical element 4 is put on the lower die 2. Subsequently, thesecond optical element material 5 is put on the first optical element 4,and then, an upper die 1 is inserted into the sleeve die 3 so as to beplaced on the second optical element material 5. When the upper die 1and the lower die 2 are inserted into the sleeve die 3, the centers ofthe upper die 1 and the lower die 2 are aligned with each other.

With the use of the upper die 1, the lower die 2, and the sleeve die 3,the second optical element material 5 is pressed against the firstoptical element 4 under a condition free from restriction to the secondoptical element material 5 in the direction orthogonal to a direction inwhich the second optical element material 5 is pressed while beingheated at a temperature incapable of deforming the first optical element4 and capable of deforming the second optical element material 5. Inother words, the second optical element material 5 is heated and pressedwith the upper face 5 a and the lower face 5 b thereof being in contactwith the upper die 1 and the first optical element 4, respectively, andwith the entirety of the curved outer peripheral face 5 c being out ofcontact with the sleeve die 3. In the present embodiment, the heatingtemperature is 485° C., the applied pressure is 200 kgf/cm², and thepressing period of time is 40 seconds. Thus, the second optical element6 is formed, as shown in FIG. 2( b).

The second optical element material 5 herein is heated and pressed undera condition that its free deformation is restricted in the verticaldirection, namely, toward the upper face 5 a and the lower face 5 b bythe upper die 1 and the lower die 2 while being allowed in thehorizontal direction, namely, outward from the curved outer peripheralface 5 c. Whereby, the curved outer peripheral face 8 of the secondoptical element 6 is formed into a substantially curved form in section.

Thus, the bonded optical element 7 is formed in which the second opticalelement 6 is bonded to and integrated with the upper face of the firstoptical element 4. The thus obtained bonded optical element 7 isexcellent in quality with no breakage, no crack, and no opacity.

As described above, in the present embodiment, the second opticalelement material 5 is pressed against the first optical element 4 undera condition free from restriction to the second optical element material5 in the direction orthogonal to the direction in which the secondoptical element material 5 is pressed, thereby sufficiently absorbingvariations in thickness and weight of the first and second opticalelements 4, 6 to set the center thickness of the bonded optical element7 to be almost equal to the designed value.

In consideration of variations accompanied by mass production of thebonded optical element 7, a bonded optical element was formed actuallyby using the first optical element 4 and the second optical elementmaterial 5 having thicknesses each being 20 μm thicker than therespective designed values. As a result, the center thickness of thethus formed bonded optical element 7 was almost equal to the designedvalue. Accordingly, it can be said that pressing the second opticalelement material 5 under a condition free from restriction to the secondoptical element material 5 in the horizontal direction leads toattainment of a highly accurate center thickness of the bonded opticalelement 7.

Further, when the second optical element material 5 is pressed with aspace left between the curved outer peripheral face 5 c and the sleevedie 3, the second optical element material 5 is prevented from creepingup to the upper die 1 to suppress breakage of the dies and adhesion ofglass to the dies.

OTHER EMBODIMENTS

The outer diameter and the thickness of each of the first and secondoptical elements 4, 6 may be arbitrary values different from the valuesin the above embodiment.

As well, the hating temperature, the applied pressure, the pressingperiod of time may be arbitrary values different from the values in theabove embodiment.

The first optical element 4 and the second optical element 6 are aspherical biconcave lens and a biconvex lens, respectively, in the aboveembodiment, but the present invention is not limited thereto and ispossible that a first optical element 10 and a second optical element 11are a spherical biconvex lens and a spherical concave lens,respectively, as shown in FIG. 3. Alternatively, one or both of thefaces opposite to the bonded faces of the first and second opticalelements may be aspheric.

The first optical element 4 is formed by polishing in the aboveembodiment, but the present invention is not limited thereto and a firstoptical element 12 may be formed by press forming, as shown in FIG. 4,for example.

In the above embodiment, a third optical element may be further providedwhich is bonded to and integrated with the first or second opticalelement. For example, as shown in FIG. 5, a second optical element 15and a third optical element 16 are directly bonded to and integratedwith the upper face and the lower face of a first optical element 14,respectively, by the same manner as in the above embodiment. In sodoing, the second and third optical elements 15, 16 may be formedsimultaneously or separately.

The first and second optical elements 4, 6 are made of theaforementioned glass materials in the above embodiment, but the presentinvention is not limited thereto and they may be made of glass materialsdifferent from the glass materials in the above embodiment or may bemade of plastic. Wherein, the first and second optical elements 4, 6 arepreferably made of glass materials. The glass-made first and secondoptical elements 4, 6 bring the bonded optical element 7 to have ahighly accurate form, high heat resistance, high mechanical durability,and high homogeneity. The same is applicable to the aforementionedbonded optical element 7 including the third optical element.

In the above embodiment, at least one of the optical elements may becentered after bonding and integrating the optical elements. Forexample, as shown in FIG. 6, after first and second optical elements 17,18 are bonded to and integrated with each other, only the second opticalelement 18 may be centered. Alternatively, only the first opticalelement 17 may be centered or both the first and second elements 17, 18may be centered. Centering in this way improves the eccentricityaccuracy of the bonded optical element 7. The same is applicable to theaforementioned bonded optical element 7 including the third opticalelement.

The first optical element 4 is a spherical lens in the above embodiment,but a lens face part (an optically functioning face) 22 of the bondedface of a first optical element 20 to a second optical element 21 may beaspheric, as shown in FIG. 7. In this case, a lens face part 23 of thebonded face of the second optical element 21 to the first opticalelement 21 is aspheric also. Each of the lens face parts 22, 23, whichinclude an optically effective face, is a curved face ranging up to theupper face (the edge face) of the edge portion. When the lens face parts22, 23 of the bonded faces of the bonded optical element 7 are set to beaspheric in this way, the degree of freedom in designing an opticalsystem increases, thereby leading to multifunction and size reduction ofan optical system. In the case where the first optical element 20 isformed by press forming herein, the lens face part 22 thereof can beeasily formed so as to be aspheric. The same is applicable to theaforementioned bonded optical element 7 including the third opticalelement.

Furthermore, the direction in which the second optical element materialis pressed agrees with the vertical direction in the above embodiment,but the present invention is not limited thereto.

The present invention is not limited to the above embodiments and can bereduced in practice in various ways without deviating from the sprit andthe main subject matter of the present invention.

As described above, the above embodiments are mere examples in everyaspect and must not be construed limitedly. The scope of the presentinvention is indicated by the attached claims and is not limited to thespecification. Further, any of variations and modifications belonging tothe equivalent scope of the claims fall in the scope of the presentinvention.

INDUSTRIAL APPLICABILITY

As described above, the present invention is applicable for the purposeof highly accurately managing the center thickness of a bonded opticalelement and the like.

1. A bonded optical element comprising a first optical element and a second optical element bonded to each other by forming the second optical element by heating and pressing against the first optical element a second optical element material, wherein the second optical element is formed by pressing the second optical element material against the first optical element under a condition free from restriction to the second optical element material in a direction orthogonal to a direction in which the second optical element material is pressed.
 2. The bonded optical element of claim 1, wherein the second optical element material is pressed against the first optical element under the condition free from restriction to the second optical element material in the direction orthogonal to the direction in which the second optical element material is pressed to cause the second optical element to have a curved outer peripheral face in a substantially curved form in section.
 3. The bonded optical element of claim 1, further comprising: a third optical element bonded to the first or second optical element.
 4. The bonded optical element of claim 1, wherein the optical elements are made of glass materials.
 5. The bonded optical element of claim 1, wherein at least one of the optical elements is centered after the optical elements are bonded to each other.
 6. The bonded optical element of claim 1, wherein a lens face part of a bonded face of the first optical element is aspheric.
 7. A method for manufacturing a bonded optical element in which a first optical element and a second optical element are bonded to each other by forming the second optical element by heating and pressing against the first optical element a second optical element material, the method comprising the step of: forming the second optical element by pressing the second optical element material against the first optical element under a condition free from restriction to the second optical element material in a direction orthogonal to a direction in which the second optical element material is pressed.
 8. The method of claim 7, wherein the second optical element is formed by pressing the second optical element material against the first optical element under the condition free from restriction to the second optical element material in the direction orthogonal to the direction in which the second optical element material is pressed to cause the second optical element to have a curved outer peripheral face in a substantially curved form in section.
 9. The method of claim 7, further comprising the step of: bonding a third optical element to the first or second optical element.
 10. The method of claim 7, comprising the step of: preparing glass materials as the optical element and the optical element material.
 11. The method of claim 7, further comprising the step of: centering at least one of the optical elements after the optical elements are bonded to each other.
 12. The method of claim 7, wherein a lens face part of a bonded face of the first optical element is aspheric. 